1. Field of the Invention
This invention relates to the field of data processing systems. More particularly, this invention relates to the conditional execution of program instructions within such data processing systems.
2. Background
It is known to provide data processing systems responsive to instructions sets which include program instructions that are conditionally executed or not executed depending upon the results of previous processing operations. As an example, the ARM 32-bit instruction set is fully conditional in that all instructions include a 4-bit condition code field specifying which conditions will suppress or allow execution.
Whilst the fully conditional 32-bit ARM instruction set is powerful in its conditional behaviour, the 4-bits dedicated to condition codes within each instruction represents a significant proportion of the bit coding space of the instruction set. In many cases the full capabilities of the condition codes are not utilised and the 4-bit dedicated to these condition codes are not well used.
It is also known to provide non-conditional instruction sets such as the Thumb instruction set supported by ARM processors. Not dedicating any instruction bits space to condition codes allows Thumb instructions to be smaller thereby improving code density. However, a disadvantage of this approach is that the number of instructions needed to perform certain operations maybe greater and processing consequently slower without the ability to exploit the condition codes of the 32-bit ARM instruction set.
It is known from other processors such as the PA-RISC processor, to provide instructions which can annul the next instruction depending upon their result. An example of such an instruction would be a compare operation which served to either suppress or allow the next following instruction to execute depending upon whether the numbers compared were equal or not equal. Whilst such annul next instruction types are useful, they have limited flexibility in that they only serve to annul the next instruction dependent upon their own processing.